The present invention relates to a mechanically driven roller vane pump used for operating an automatic transmission for motor vehicles, in particular a continuously variable transmission (CVT).
The roller vane pump comprises a pump housing, a carrier having a substantially circular cross section and being located in the interior of the pump housing, said carrier being rotatable by means of a drive shaft, a ring shaped cam ring having a non-circular inner surface and surrounding the carrier in radial direction, and substantially cylindrical roller elements being slidably provided in slots on the periphery of the carrier. The roller vane pump further comprises at least one feed aperture and at least one discharge aperture, said apertures being arranged in the pump housing and having a substantially elongated shape, the long axes of said apertures extending in a substantially tangential direction. Said apertures are divided into an inner aperture and an outer aperture by a narrow ridge. Said ridge supports the roller elements in axial direction.
On rotation of the carrier, the roller elements interact with an inner surface of the cam ring along contact lines there between, under influence of a pressure and/or a centrifugal force. The spaces between the pump housing, the carrier, the cam ring and the roller elements define pump chambers, which may arrive into communication with hydraulic channels in the pump housing through the feed apertures and the discharge apertures for allowing a flow of fluid to or from the pump chambers. The pump chambers are divided into cam chambers and carrier chambers, said cam chambers ranging from tangential centre planes of the roller elements radially outward, and said carrier chambers ranging from tangential centre planes of the roller elements radially inward, in which the tangential centre plane of a roller element is a plane that extends through the centre line of the cylindrical roller element in axial direction as well as in tangential direction, in other words, a plane that extends substantially parallel to the periphery of the carrier. Each roller element is associated with a leading cam chamber and a trailing cam chamber, the leading cam chamber ranging from a radial centre plane of the roller element in rotational direction, and the trailing cam chamber ranging from said radial centre plane of the roller element in anti-rotational direction, in which the radial centre plane of a roller element is a plane that extends in axial direction through the centre line of the cylindrical roller element as well as through the contact line between the roller element and the cam ring. Thus, a cam chamber that extends between two roller elements acts simultaneously as leading cam chamber for the roller element in anti-rotational direction and as trailing cam chamber for the roller element in rotational direction. As each roller element is associated with on the one side a carrier chamber and on the other side a leading cam chamber and a trailing cam chamber, each carrier chamber corresponds with a leading cam chamber and a trailing cam chamber.
The radius of curvature of the inner surface of the cam ring changes along the circumference of the cam ring. As a result, the volume of each pump chamber varies during rotation of the carrier, in connection with the tangential position of the pump chamber. When the volume of a pump chamber increases, the pressure in that chamber, i.e. the feed pressure, decreases, and fluid is drawn from a reservoir through hydraulic feed channels and the feed apertures into the pump chamber. Consequently, the tangential position of the feed apertures relative to the cam ring is such that the pump chambers arrive into contact with the feed apertures when the pump chamber volume increases. When the volume of a pump chamber decreases, fluid is discharged from said pump chamber through the discharge apertures and hydraulic discharge channels to a user of pressurized fluid, whereby a higher pressure, i.e. a discharge pressure, may be effected. Consequently, the tangential position of the discharge apertures relative to the cam ring is such that the pump chambers arrive into contact with the discharge apertures when the pump chamber volume decreases.
A roller vane pump as described in the above is known from the European patent 0.921.314 and is suitable for pumping automatic transmission fluid in hydraulically controlled and/or operated automatic transmissions for motor vehicles, in particular continuously variable transmissions. In a continuously variable transmission (CVT), such as a belt-and-pulley type CVT, a large flow of fluid may be required for control of the transmission ratio. Since the pump is driven by a shaft drivingly connected to the engine shaft, the pump is designed to be able to provide a desired pump yield, i.e. a desired flow rate at a desired pressure, even at the lowest rotational speed of the engine. On the other hand, the pump is also able to reliably cope with the extremely high pump yield that will be provided at the uppermost rotational speed of the vehicle engine.
Although the known roller vane pump functions satisfactory per se, it possesses the drawback that cavitation is apt to occur inside the known roller vane pump, amounting both to wear of pump parts and to noise generated by the pump.
At high rotational speeds of the vehicle engine, a high pump yield is provided, i.e. a large flow of fluid is discharged. To enable such large discharge flow, an equally large feed flow must be drawn to the pump. As the flow of the fluid through a feed aperture to a pump chamber is dependent of the level of an underpressure effected in the pump chamber and of the surface area of the respective aperture, wherein the surface area of the apertures in a pump is a constant, the underpressure required for drawing such large feed flow will be large as well, so that cavitation is apt to occur.
An aim of the invention is to reduce the noise generated by the pump and to reduce the wear of pump parts. This aim is, according to the insight underlying the present invention, achieved in enlarging the surface area of the apertures through which fluid is allowed to flow to and from the pump chambers. In particular, this aim is achieved in providing for a modified shape of the ridge, wherein at least one of an inner surface and an outer surface of the ridge extends substantially parallel to the cam ring surface over a substantial part of the tangential dimension of said ridge. At the same pump yield, a larger surface area of the apertures means a less extreme underpressure, i.e. a higher feed pressure in a pump chamber when in communication with the feed channels, which results in a reduction of noise generated by the pump and in a reduction of wear of pump parts. The surface area of the apertures in the known roller vane pump is smaller than said surface area in the pump according to the present invention, because in the known pump the ridge surfaces have the shape of a segment of a circle and extend substantially parallel to the circular periphery of the carrier. With this known shape of the ridge, the distribution of the flows of fluid among the inner aperture and the outer aperture is not well-balanced and not optimal for most tangential positions of the carrier.
In a preferred embodiment of the pump according to the invention, the surface area of the apertures is at a maximum, because the ridges are located such that the radial distance between the centre lines of the ridges and the cam ring surface is substantially equal to the radius of the roller elements. This configuration also provides an optimal axial support of the roller elements by supporting the roller elements centrally and over a maximum possible surface area of the roller elements for a given radial width of the ridges.
Another drawback of the known roller vane pump is that the roller elements are known to intermittently loose contact with the cam ring surface, which is particularly undesirable at the instance the fluid pressure in a pump chamber associated with a roller element changes from the feed pressure to the discharge pressure and vice versa. This undesired loss of contact amounts to wear of pump parts, noise generated by the pump and a decrease in pump efficiency.
The roller element looses contact with the cam ring surface when a force generated by a pressure difference between the carrier chamber and the corresponding cam chambers is directed radially inward and has a higher value than the centrifugal force, which is directed radially outward. The roller element then moves in radial inward direction under influence of a resultant force, which is directed radially inward, and looses contact with the cam ring surface. Such an undesired movement occurs when the fluid pressure in the carrier chamber is lower than the mean fluid pressure of the corresponding cam chambers.
An aim of the invention is to reduce the noise generated by the pump, to reduce the wear of pump parts, and to obtain higher pump efficiency. This aim is, according to the insight underlying the present invention, achieved in taking constructional measurements to ensure that the fluid pressure in the carrier chamber is always higher than, or at least equal to the mean fluid pressure of the corresponding cam chambers. In particular, this aim is achieved in providing for a modified arrangement of the apertures, wherein the feed aperture is shaped such that the leading cam chamber arrives into communication with the outer feed aperture before the corresponding carrier chamber arrives into communication with the inner feed aperture; and wherein the discharge aperture is shaped such that the carrier chamber arrives into communication with the inner discharge aperture before the corresponding leading cam chamber arrives into communication with the outer discharge aperture. In addition, the feed aperture can be shaped such that the communication between the carrier chamber and the inner feed aperture is cut off before the communication between the leading cam chamber and the outer feed aperture is cut off. Moreover, the discharge aperture can be shaped such that the communication between the leading cam chamber and the outer discharge aperture is cut off before the communication between the carrier chamber and the inner discharge aperture is cut off. Contrary to these aperture shapes, the aperture shapes in the known roller vane pump are such that the carrier chamber arrives into communication with the inner feed aperture before the corresponding leading cam chamber arrives into communication with the outer feed aperture, and the leading cam chamber arrives into communication with the outer discharge aperture before the corresponding carrier chamber arrives into communication with the inner discharge aperture. Also, the feed aperture is shaped such that the communication between the carrier chamber and the inner feed aperture and the communication between the corresponding trailing cam chamber and the outer feed aperture are cut off at approximately the same moment. In the roller vane pump according to the present invention the pressure in the carrier chamber will not become lower than the mean pressure of the corresponding cam chambers, so that the resultant force on the roller elements as a result of these pressures will not be oriented in a radially inward direction. The roller elements will thus maintain their interaction with the cam ring surface which results in a reduction of noise generated by the pump, a reduction of wear of pump parts and an increase in pump efficiency.
Another drawback of the known roller vane pump is that a chamber arrives in communication with an aperture rather suddenly, because the radial dimension of the apertures is almost immediately at a maximum. Consequently, the fluid pressure in the pump chamber changes abruptly, which results both in wear of pump parts as well as a high level of the noise generated by the pump.
An aim of the invention is to reduce the noise generated by the pump as well as the wear of pump parts. This aim is achieved providing an aperture with an end part extending in anti-rotational direction, such that at the location of said end part a pump chamber arrives into communication with the aperture through an opening there between, which opening has a constant radial width that is significantly less than that of the widest part of the aperture. Accordingly, the pressure in a pump chamber may be brought to the level prevailing in the hydraulic channel associated with the respective aperture in a defined and gradual manner, e.g. substantially without dynamic effects causing pressure fluctuations or vibrations, essentially before the fluid flow to or from the pump chamber starts.
It is remarked that an aperture with an end part extending in anti-rotational direction, at least a part of which has a significantly smaller radial dimension as the main part of the aperture, however, with a radial dimension which decreases in anti-rotational direction are well known in the technical field of rotary pump design, for instance from FR-A-2.095.994 or EP-A-0.200.294. According to the invention it was found that such known end part geometry is not optimal for the present type pump which is operable in a wide range of rotational speeds of the carrier. Analysis of the pressure changes occurring in the pump as a function of said rotational speed revealed that, although with the known end part geometry said pressure changes may be the most gradual at a certain rotational speed, at other rotational speeds they are still relatively abrupt. Using the end part geometry according to the invention, it is advantageously achieved that said pressure changes are sufficiently gradual for most rotational speeds of the carrier.
In a preferred embodiment of the pump according to the present invention, said end part is a slit formed by co-operation between the cam ring and the outer feed aperture, wherein the end part of the outer feed aperture is shaped such that it overlaps the cam ring in axial direction. This configuration may be manufactured easily and is therefore relatively cheap. It is remarked that the configuration of cam ring and outer feed aperture may alternatively also be adopted to cheaply form a slit-shaped end part having a radial width which decreases in anti-rotational direction.
In another preferred embodiment of the pump according to the present invention, said end part is a groove formed by a recess in the pump housing adjoining the carrier. The carrier chamber arrives into communication with the discharge aperture at the location of the groove, before arriving into communication with a part of the discharge aperture having a significantly larger radial dimension. With this construction, a less abrupt pressure increase in the carrier chamber is obtained than with the known construction wherein the carrier chamber arrives directly into communication with a discharge aperture having a relatively large radial dimension. With respect to the known constructions incorporating an end-part having an radial width which increases in the direction of rotation of the carrier, the pump performance is improved in that for a substantial part of the range of rotational speeds of the carrier gradual pressure changes in the pump chamber are obtained.
It was found that a groove having a rectangular cross section is particularly suitable for a CVT-like application of the roller vane pump, wherein the pump must be able to cope with high pressures and a widely variable rotational speed of the carrier. Such a groove preferably has a depth in axial direction that increases in the direction of rotation of the carrier.
Due to the gradual pressure changes that will be achieved with the measure according to the invention, the noise generated by the pump is reduced as is the wear of pump parts.
In a preferred embodiment of the pump according to the present invention, the pump is provided with a gap between the roller elements and the carrier in tangential direction. The gap forms a channel through which corresponding cam and carrier chambers are in communication. These small channels contribute to a smoothing of the pressure differences between the carrier chamber and the cam chamber. If adopted in combination with an aperture having an end part according to the present invention, only one of the inner or the outer aperture needs to be provided with such end part. The pressure in a part of the pump chamber not associated with the said only one of the inner or the outer aperture is gradually changed through communication through said gap. It may thus advantageously be achieved that the fluid pressure in the chamber which is initially not in communication with an aperture, still is changed to some degree in accordance with the chamber which is in communication with an aperture, resulting in a smaller fluid pressure increase or decrease in the firstly mentioned chamber when it does arrive into communication with the aperture. In a preferred embodiment of the invention the width of the gap in tangential direction is dimensioned such that the rate at which the fluid pressure changes in the cam chamber during operation substantially corresponds to that in the carrier chamber. According to the invention the width of the gap in tangential direction may also be dimensioned such, that it corresponds to a minimum width in tangential direction required for allowing said pressure difference to become approximately zero. It is remarked, that taking a minimum width of the gap for achieving the above-mentioned requirements is highly advantageous, because then the amount of tangential movement and the tangential speed of the roller elements is limited, thereby limiting pump noise and wear. A width in tangential direction having a value in the range from 0.03 mm to 0.18 mm was found to be particularly suitable. In a presently favoured design of the roller vane pump such range corresponds to about 0.5 to 2.5 percent of a diameter of the roller element.